Field of the Invention
This invention relates to a sensor head assembly capable of detecting forces down to a level of 10.sup.-12 N (Newton) Forces of that order of magnitude may occur, for example, between individual atoms or molecules if they are brought closely together, yet without touching. The sensor head of the present invention can, therefore, find application in the Scanning Atomic Force Microscope as proposed by G. Binnig et al in 1985.
Description of the Related Art
While one embodiment of an atomic force microscope is described in copending U.S. application Ser. No. 892,977, filed Aug. 4, 1986, now U.S. Pat. No. .varies.,724,318 and assigned to the present assignee, a brief description of its features will enhance the understanding of the present invention to be described below. An atomic force microscope typically comprises a sharply pointed tip of a hard material mounted at the free end of an elastic cantilever beam, and a detector for sensing the deflection said cantilever beam undergoes when the tip is brought close to an atom or molecule. Since the interatomic forces occurring between the tip and the atom nextmost to the tip's apex are so small, so is the ensuing deflection of the cantilever beam.
There are several methods known in the art for detecting deflections on the order of fractions of a nanometer. It may, therefore, suffice to refer to just one of them. It is known that the tunneling current flowing across a gap between a conductor and a sharp pointed conductive tip is exponentially dependent on the width of the gap. If, therefore, the cantilever beam carries a dielectric tip and, on its face opposite from the dielectric tip it carries, is electrically conductive and a sharp metal tip is maintained at a tunneling distance from the cantilever, the variation of the tunneling current will precisely indicate the deflection the cantilever performs as its tip is confronted with the external atom or molecule. For a description of a Scanning Tunneling Microscope which makes use of this dependence of the tunneling current on the gap width, reference is made to U.S. Pat. No. 4,343,993.
From the foregoing description of an atom force microscope it will be clear to those skilled in the art that the most critical part of an atomic force microscope is the sensor head assembly comprising the cantilever beam and the dielectric tip it carries. Because this is difficult to achieve, the atomic force microscope is a very recent development there being no other beams far known which can detect forces of magnitude less than 10.sup.-12 N. No efforts have been reported to develop a sensor head assembly that meets the requirement of detecting forces of such small magnitude. In the embodiment described in accordance with aforementioned U.S. appln. Ser. No. 892,977, now U.S. Pat. No. 4,724,318, and also in an article entitled "Atomic Force Microscope" by G. Binnig, C. F. Quate and Ch. Gerber, Phys. Rev. Lett. Vol. 56, No. 9, 1986, pp. 930-933, the cantilever beam was a gold foil of about 25 .mu.m thickness, 800 .mu.m length and 250 .mu.m width, to which a diamond tip was glued. It will be obvious to those skilled in the art that the fabrication and handling of a cantilever beam of that sort are extremely delicate and prone to low yield.
It is, therefore, one object of the present invention to propose a micromechanical atomic force sensor head which is relatively easy to manufacture and handle, and which is very inexpensive when produced in quantity. An important novel feature of the proposed atomic force sensor head is that it incorporates a cantilever beam and tunnel gap in one monolithic device.
Prior to starting a description of the details of the present invention, it may be useful to briefly explain the meaning of `micromechanical`. Microminiaturization of electronic devices has led to a comprehensive study of many properties of the element silicon and it has ben found that, besides its advantages as a semiconductor, it has a number of mechanical characteristics which make it very useful in the design of mechanical structures. In particular, since the electronic circuits manufactured on silicon wafers require miniaturization, it has been learned how to construct mechanical silicon parts with very small dimensions using essentially the same manufacturing techniques (such as lithography, epitaxy, that have been used to make electronic circuits. Therefore, the term `micromechanical` is used to described miniaturized mechanical structures made of silicon and its compounds.
Further information on micromechanics may be obtained from the following publications:
K. E. Peterson, Dynamic Micromechanics on Silicon: Technique and Devices, IEEE Trans. on Electron Devices Vol. ED-25, No. 10, October 1978, pp. 1241...1250; PA1 J. B. Angell, S. C. Terry, P. W. Barth, Silicon Micromechanical Devices, Scientific American, Vol. 248, No. 4, 1983, pp. 36...47; PA1 A. Heuberger, Mikromechanik - Der Chip lernt fuhlen, VDI Machrichten-Magazin 4/85, pp. 34-35.